efi: make efi_mem_type() and efi_mem_attributes() work on Xen PV

efi: make efi_mem_type() and efi_mem_attributes() work on Xen PV

Xen doesn't give direct access to the EFI memory map, but provides a
hypercall interface for it. efi_mem_desc_lookup() was already adjusted
in aca1d27ac38a "efi: xen: Implement memory descriptor lookup based on
hypercall" to (optionally) use it. Now make efi_mem_type() and
efi_mem_attributes() use common efi_mem_desc_lookup() too.
This also reduces code duplication a bit.
efi_mem_type() retains separate check for -ENOTSUPP error case (even
though no caller seems to rely on this currently).

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
[ardb: Drop erroneous 'const' qualifier]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>